Backer inserts for blocking backwall water jet strikes

ABSTRACT

A method of drilling holes in an component, such as a turbine airfoil or bucket platform, the method including disposing a protective insert including a water jet blocking material adjacent a backside of the structure to be drilled and water jet drilling at least one hole in the structure, through to the backside. In one embodiment, the jet blocking material is comprised of carbide or a similar material that is more resistant to water jet drilling forces than the structure being drilled.

BACKGROUND OF THE INVENTION

[0001] Airfoils of turbine blades and vanes of gas turbine engines oftenrequire a complex cooling scheme in which cooling air flows through theairfoil and is then discharged through carefully configured coolingholes in the side wall of the airfoil and/or its associated structures.The performance of a turbine airfoil is directly related to the abilityto provide uniform cooling of its external surfaces. Consequently,control of cooling hole size and shape is critical in many turbineairfoil designs, because the size and shape of the opening determinesthe amount of flow exiting a given opening, its distribution across thesurface of the component, and the overall flow distribution within thecooling circuit that contains the opening. Other factors, such as backflow margin (the pressure differential between the cooling air exitingthe cooling holes and combustion gas impinging on the airfoil) are alsoaffected by variations in opening size.

[0002] Conventional hole drilling techniques include laser machining andelectrical-discharge machining (EDM). These techniques yield airfoilcastings with dimensionally correct openings in order to repeatablycontrol opening size.

[0003] Water jet drilling is another versatile drilling method forprecision drilling operations. However, conventional water jet drillingis primarily performed on structures that do not have a shallow dropthrough region. This is due to the physical limitations of being able tostop the drilling jet before it hits an opposing surface. Whilesacrifices could be made to allow for the opposing wall to bepart-drilled, this would necessarily result in a decrease in part lifeand field performance. Such a sacrifice is illogical where otherdrilling techniques that do not decrease part life are available. Thus,water jet drilling is generally considered unsuitable for drillingnozzles and buckets. Nevertheless it would be advantageous to provide amethod for drilling airfoil cavities with a water jet in a manner thatavoids damage to an adjacent wall, once the hole has been drilledthrough and before the application of the jet is terminated.

BRIEF DESCRIPTION OF THE INVENTION

[0004] The invention is embodied in a method for water jet drillingstructures, such as nozzles and buckets used in gas turbines, whereinthe opposite wall and/or adjacent structures are shielded from the waterjet by providing a backer insert as a jet-stop to prevent unwantederosion or drilling of the airfoil structure.

[0005] In one embodiment of the invention, in order to increase thedurability of the backer, the backer is at least one of formed from orcoated with a material that wears at a slow rate. More specifically, anywater jet blocking material that is more resistant to water jet drillingforces than the material of the structure being drilled may be used toadvantage in a method and/or insert embodying the invention. Anexemplary material that may be adopted for the backer of the inventionis carbide. Carbide by its physical nature is slow to wear, thusoffering the durability required in part to part processing in anymanufacturing environment.

[0006] According to a first aspect of the invention, a method isprovided for drilling holes in a wall of a component having a hollowinterior cavity, the method comprising disposing a backer insertcomprising a water jet blocking material in the hollow interior cavityadjacent to a back surface of the wall to be drilled; water jet drillingat least one hole in the wall through to the hollow interior cavity; andremoving the backer insert.

[0007] In one embodiment, the backer insert is formed from or coatedwith a blocking material, such as carbide, that is more resistant towater jet drilling forces that the material of the component wall.

[0008] According to another aspect of the invention, a backer insert isprovided for being disposed in a cavity of a gas turbine component tointercept and disperse a water jet for drilling of a hole through a wallof the cavity, the backer insert comprising an insert component having aconfiguration generally corresponding but smaller than to aconfiguration of a back surface of the wall of the cavity. The insertcomponent is formed from or coated with a water jet blocking material.In one embodiment, the blocking material is more resistant to water jetdrilling forces that the material of the wall being drilled.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] These, as well as other objects and advantages of this invention,will be more completely understood and appreciated by careful study ofthe following more detailed description of the presently preferredexemplary embodiments of the invention taken in conjunction with theaccompanying drawings, in which:

[0010]FIG. 1 is a schematic perspective view illustrating backercomponents provided as inserts to a nozzle airfoil;

[0011]FIG. 2 is a cross-sectional view of a nozzle airfoil showing firstand second backer insert components in place in respective cavities ofthe nozzle airfoil consistent with the backer insert placement shown inFIG. 1;

[0012]FIG. 3 is an elevational view of a bucket having a backer insertcomponent provided in the pocket area of the bucket shank;

[0013]FIG. 4 is an end elevational view, partly in cross-section of thebucket of FIG. 3 showing the backer insert exploded away from the bucketshank to illustrate its placement therein;

[0014]FIG. 5 is a schematic cross-sectional view taken along lines 5-5of FIG. 3 showing the backer insert exploded away from the bucket shankpocket according to an embodiment of the invention; and

[0015]FIG. 6 is a cross-sectional view taken along lines 5-5 of FIG. 3illustrating the backer insert in position in the bucket shank pocket.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The invention provides a method and backer insert to allow theuse of a water jet process to drill cooling holes in airfoil walls andvane platforms by using a backer insert having a configuration generallycorresponding to the back surface of the structure to be drilled, tointercept the water jet on hole completion, to prevent damage toadjacent structures.

[0017] In the embodiment illustrated in FIG. 2, a nozzle 10 is beingdrilled using an otherwise conventional water jet apparatusschematically shown at 12. To prevent the water jet from eroded ordrilling the opposite wall of the airfoil, backer inserts 14,16 areinserted into the respective cavity 18, 20 of the nozzle 10 forintercepting the drilling jets 22, 24 during the drilling operation. Inthe embodiment illustrated in FIGS. 1 and 2, the backing inserts arerespectively placed inside the part during the drilling operation andremoved on completion of the drilling process with respect to thatcavity, or with respect to the nozzle.

[0018] As illustrated in FIG. 2, the backing inserts 14, 16 aremeasurably smaller than their respective cavity 18, 20 to define a smallgap 26, 28 between the backer insert and the cavity wall, to allow forthe disbursal of the water jet and any added abrasive material, such asgarnet, to allow a clean through hole into the cavity to be formed. Thebacker insert is formed from or coated with a material that is slow towear on exposure to the water jet. Forming the backer insert fromCarbide, as schematically shown at 30 and/or coating a substrate 32 withCarbide, as schematically shown by layer 34, to form the backer insertadvantageously meets this criteria.

[0019] As noted, the function of the backer insert is to limitpenetration of the water jet into the interior of the cavity. As such,the backer insert is advantageously placed adjacent to but spaced fromthe backsurface of the structure to be drilled, as described above. Itis not necessary, however, for the backer insert to otherwise fill thecavity and/or to be a solid component. Thus, as an alternative toproviding an insert that effectively fills the cavity but for the gapfrom the peripheral walls, as illustrated and described above, a hollowinsert may be provided that is shaped, for example, as a conventionalhollow impingement insert, but with imperforate wall(s) and formed froma water jet resistant material, as mentioned above. As a furtheralternative, the backer insert may be in the form of insert rod(s),tube(s), or plate(s) adapted to be disposed adjacent to but spaced fromthe inner wall of the structure to be drilled.

[0020] Where water jet drilling is to be performed sequentially orsimultaneously on opposite side walls of the nozzle cavity, the backerinsert advantageously includes wall members disposed adjacent eachsidewall of the nozzle. This can be accomplished by providing agenerally solid or hollow insert in the configuration of an impingementinsert as described above. In the alternative, first and second backerinsert plates may be provided and interconnected by a scaffold or othertransverse support structure to define e.g., an I beam type assembly. Byproviding plates that are pivotally attached to the transverse supportstructure, and/or by providing a transverse support structure that canbe dimensionally adjusted to alter a gap between the backer insertplates, further versatility can be afforded for using a single backerinsert in a variety of nozzle cavities. It is to be appreciated,however, that the material and weight savings achieved by providing ahollow insert or supported plates rather than a solid insert is at theexpense of durability and thus life span of the component.

[0021] As illustrated in FIGS. 3-6, the process of the invention can beused as well for water jet drilling of holes in structures borderingnon-cavity pockets, such as for the formation of bucket platform holes,to prevent unwanted strike of the pocket area of the shank. Morespecifically, a typical bucket structure 50 is schematically illustratedin the elevational views of FIGS. 3 and 4, except that a backer insert64 is illustrated as selectively disposed in a pocket area 72 of thebucket shank. Thus, referring particularly to FIGS. 3 and 4 there isillustrated a turbine bucket 50 that includes an airfoil 52 mounted on aplatform 54 that is in turn carried by a shank 56. The radial inner endof the shank carries a dovetail 58 for coupling the blade to a turbinewheel (not shown). In the illustrated embodiment, the airfoil has acompound curvature with suction and pressure sides 60, 62. As such, theturbine blade structure generally corresponds to that disclosed in U.S.Pat. No. 5,980,209, the entire disclosure of which is incorporatedherein by this reference.

[0022] As illustrated in FIG. 3, cooling holes as schematically shown at66 may be formed to advantage through the platform 54 in the vicinity ofthe pocket area on one or each side of the airfoil 52, at its base. Inan embodiment of the invention, the cooling holes are formed usingdrilling water jet(s) 68 generated by a water jet apparatus,schematically shown at 70. To protect the peripheral wall of the pocketarea from potential damage from the drilling water jet, a backer insert64 is selectively inserted into the pocket area 72 during the drillingoperation. As illustrated in FIG. 3, the backer insert 64 may take theform of a plate or shell, or a solid insert, that is shaped to generallycorrespond to the shape of the wall of the pocket area 72. However, asillustrated in FIGS. 5 and 6, to facilitate flow of the fluid and debrisduring and following the water jet drilling process, a gap 74 isadvantageously defined between the backer insert and the associatedpocket area.

[0023] As such, the water jets for drilling the cooling holes, asschematically shown by dash-dot lines 68, will engage and be dissipatedby the backer insert upon penetration through the bucket platform todefine the air-cooling holes 66.

[0024] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of forming drilled holes in a wall of acomponent having a hollow interior cavity, said method comprising:disposing a backer insert comprising a water jet blocking material insaid hollow interior cavity adjacent to a back surface of said wall tobe drilled; water jet drilling at least one hole in the wall through tothe hollow interior cavity; and removing the backer insert.
 2. A methodas claimed in claim 1, wherein said backer insert is one of formed fromand coated with said blocking material.
 3. A method as claimed in claim2, wherein said blocking material is more resistant to water jetdrilling forces than a material of said wall.
 4. A method as claimed inclaim 3, wherein said blocking material comprises carbide.
 5. A methodas claimed in claim 1, wherein said backer insert has a configurationgenerally corresponding to said back surface.
 6. A method as claimed inclaim 1, wherein said backer insert is disposed adjacent to but spacedfrom said back surface, to define a gap for fluid to flow away from saidat least one hole.
 7. A method as claimed in claim 6, wherein saidbacker insert has a configuration generally corresponding to but smallerthan to a configuration of at least a part of said cavity so as todefine said gap between said back surface of said wall and said insert.8. A method as claimed in claim 1, wherein said backer insert issubstantially solid and fills a substantial portion of said cavity.
 9. Amethod as claimed in claim 1, wherein said component comprises a nozzleof a gas turbine.
 10. A method as claimed in claim 1, wherein said wallis a side wall of an airfoil of said nozzle, and wherein said hollowinterior cavity is a vane cavity of said airfoil.
 11. A method asclaimed in claim 1, wherein said component comprises a bucket of a gasturbine.
 12. A method as claimed in claim 1, wherein said wall is aplatform of the bucket, and said hollow interior cavity is a shankpocket of the bucket.
 13. A backer insert for being disposed in a cavityof a gas turbine component to intercept and disperse a water jet fordrilling of a hole through a wall of the cavity, comprising: an insertcomponent having a configuration generally corresponding to but smallerthan to a configuration of a back surface of the wall of the cavity,said insert component being one of formed from and coated with a waterjet blocking material, said blocking material being more resistant towater jet drilling forces than a material of said wall.
 14. A backerinsert as in claim 13, wherein said blocking material comprises carbide.15. A backer insert as in claim 13, wherein said insert component issubstantially solid and is configured to fill a substantial portion ofthe cavity.
 16. A backer insert as in claim 13, wherein said gas turbinecomponent comprises a nozzle of the gas turbine.
 17. A backer insert asin claim 16, wherein said wall is a side wall of an airfoil of thenozzle, and wherein said cavity is a vane cavity of said airfoil.
 18. Abacker insert as in claim 13, wherein said gas turbine componentcomprises a bucket of the gas turbine.
 19. A backer insert as in claim18, wherein said wall is a platform of the bucket, and said cavity is ashank pocket of the bucket.